A promoter located immediately upstream of the XLαs promoter drives expression of an antisense transcript, which extends past the exon(s) encoding NESP55 [93, 94] (Fig. 2). In mice, the mature antisense transcript (termed Nespas or Gnasas) encompasses both Nesp55 exons 1 and 2 and the intervening intron [94], while in humans, it has no overlap with exon NESP55 [93]. There are at least five distinct exons that form the human GNAS antisense transcript, but alternative splicing results in at least six variants. The largest open reading frame is predicted to encode a polypeptide of 97 amino acids that share no homology to known proteins [93]. Taken together these features suggest that the antisense transcript is non-coding. Indeed, similar to the promoter of A/B, the promoter of the GNAS antisense promoter resides in a DMR and is active exclusively on the paternal allele [65, 93, 94]; however, the antisense transcript shows biallelic expression in the adrenal and testes [65]. Consistent with allele-specific expression, the paternal antisense promoter is associated with acetylated histone and histone-3 Lys4 methylation, while the maternal promoter lacks
